Liquefied hydrocarbons such as natural gas liquids (NGL) or liquid petroleum gas (LPG) are a flammable mixture of hydrocarbon gases used as a fuel in heating appliances and vehicles. They are also increasingly used as an aerosol propellant and a refrigerant, replacing chlorofluorocarbons in an effort to reduce damage to the ozone layer.
Liquefied hydrocarbons are synthesized by refining petroleum or “wet” natural gas, and are almost entirely derived from fossil fuel sources, being manufactured during the refining of petroleum (crude oil), or extracted from petroleum or natural gas streams as they emerge from the ground.
Liquefied hydrocarbon gases may evaporate quickly at normal temperatures and pressures and are usually supplied in pressurized steel gas cylinders. These cylinders are typically filled to between 80% and 85% of their capacity to allow for thermal expansion of the contained liquid. The ratio between the volumes of the vaporized gas and the liquefied gas varies depending on composition, pressure, and temperature, but is typically around 250:1.
The liquefied hydrocarbon gases often contain a variety of acidic, gaseous contaminants, such as hydrogen sulfide, a variety of mercaptans and other diverse sulfur compounds, carbon dioxide, and carbonyl sulfide (COS). It is well known in the gas treating industry that such contaminants can be successfully removed by contacting gas or liquid hydrocarbon streams with aqueous solutions of one or more amines. Aqueous amine solutions may be either selective or non-selective in their ability to absorb particular acid gases.
After such absorption, the acidic compounds are stripped from the amines and the amines are returned to the system, except to the extent the amine compounds may have been lost in the process. It has been theorized that many different amines would provide some level of utility for removal of acid gases. As a practical matter, the amines actually in commercial use are monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), and diisopropanolamine (DIPA). For example, use of MDEA/DIPA mixtures has been reported (U.S. Pat. No. 4,808,765) for the purpose of removing H2S.
Treatment of liquefied hydrocarbon gases presents particular problems in that amines tend to be significantly soluble in gases, leading to a corresponding economic penalty due to the need to make up the lost amine(s). Many refineries use aqueous DIPA or MDEA to remove the acidic impurities from liquefied hydrocarbon gases. However, the concentration of these amines is typically limited to the range of about 2-35 weight percent of the aqueous stream in which they are supplied to the process. Operation at higher concentrations, which is desirable for capacity reasons, generally results in undesirably high levels of liquefied hydrocarbon gas contamination with amine(s).
The problem is particularly acute at refineries treating cracked (i.e., highly unsaturated) LPG. Often, the loss rate of MDEA is sufficient to negate the economic justification for substituting MDEA for DEA.
All of U.S. Pat. Nos. 5,326,385; 5,877,386; and 6,344,949 teach some type of “sweetening” of liquefied hydrocarbon gas through various processes. Further, U.S. Pat. No. 4,959,086 uses isomers of amine compounds to remove hydrogen sulfide from natural gas. Use of MDEA/DIPA mixtures has been reported (U.S. Pat. No. 4,808,765) for the purpose of removing H2S.
These publications present reasonable solutions to problems encountered when “sweetening” liquefied hydrocarbon gas through amine-acid gas processes. However, it would be highly desirable to have an amine composition which maximizes the effective amine concentration circulating in the liquefied hydrocarbon gas system, while yet minimizes the amount of amine(s) lost due to solubility in the liquefied hydrocarbon gas.